R_F_Burns is right about radio being different today. I also think that the biggest obstacle to "dayparting" having much impact is that many listeners only listen to radio that they have control over when they're in their cars. They might have a radio station playing at work, depending on where they work, but they often don't get to control the station. When they're at home, there are too many other entertainment options more appealing that just turning the radio on.
On the other hand, I disagree with his observation about "classic hits". As someone who was listening to music in the era when the classics were becoming hits, I often bought the albums that the hits were on. And once I owned the albums, I listened to the entire thing. This caused me to find additional songs from that era that I liked as much or better than the "hits" that some executive in a suit decided should be the single that got airplay. Looking back, I have a lot of personal favorite songs from that era that I cannot remember if they were radio hits or not. I only remember that I like them. When I'd make mix-tapes for in my car, and then mix-CDs when CD players replaced cassettes, I'd include all of the songs I liked, regardless of whether they were hits or not. And when other people rode in my car, they'd often comment on how much they liked the songs they never heard before, because they didn't own the same albums I did. It was the same for me when I'd hear album cuts from old albums that were "new" to my ears.
I've said this often in forums like this one. To most of us, what makes "classic rock" (or the music of the 60'/70'/ and 80's) our favorite is the basic sound of the music. When we hear music that sounds similar to our favorite songs, like a cut from an album that we never heard before, we tend to like it.